1. Introduction
This invention relates to acidic aqueous compositions for immersion coating tin over metal surfaces displaceable by tin and processes for using the same.
2. Description of the Prior Art
Immersion tin plating compositions capable of providing tin plate over a copper surface are known and disclosed, for example in U.S. Pat. Nos. 2,282,511; 2,369,620; 2,891,871; 3,303,029 and 3,917,486, all incorporated herein by reference. Such compositions typically comprise a tin salt, a complexing agent for the tin, typically a sulphur complexing agent, especially thiourea, an acid and frequently a reducing agent such as a compound of hypophosphite.
It is known that immersion tin plating compositions plate by displacement whereby theoretically a mole of stannous tin displaces 2 moles of copper thereby introducing 2 moles of copper into solution for each mole of tin plated as a consequence of the plating reaction. As plating continues, the copper over which the tin deposits becomes coated with tin and is thereby less available for displacement. Therefore, as plating proceeds, plating rate decreases until plating substantially stops with little or no further buildup of tin regardless of the amount of tin remaining in the plating composition. Consequently, such compositions are characterized by a relatively low yield of plateable tin from solution.
It is known by applicant that when thiourea is used as a complexing agent, it would be desirable to use a high molar ratio of thiourea to tin and a relatively high tin concentration--i.e., 0.2 moles per liter. This combination increases the "throwing power" of the solution. However, it is also known by applicant that under such conditions, as the ratio of thiourea to tin increases, the deposit formed becomes discolored and non-adherent and is aptly described as "black tin".